Recent advances in sensing technology have contributed to using conductivity for traditionally difficult measurements of aqueous concentrations of acidsbases and salts.
An understanding of how conductivity measurement works and today’s conductivity capabilities can prove invaluable for your next concentration measurement challenge.
Every binary solutionwhether it is watermilkor acidis comprised of ionsmost of which exhibit electrical conductivity. When the ion concentration increases or decreasesso can the conductivity. This usually provides a reliable means to measure the concentration by weight of the solution. For examplelet’s look at waterthe most abundant solution on earth. The typical ion in water is salt (NaCl). As the concentration of salt increasesso will the detectable conductivity. Because the same premise applies to other binary solutionssuch as acidsbasesor other saltsconductivity measurement is ideal for a broad range of applications throughout all process industries.
The real measure
In the broadest senseconductivity is the ability of a material to carry electric current. The basic principle by which instruments measure conductivity is simple – two plates are placed in the solutiona potential is applied across the plates (normally a sine wave voltage)and the current is measured. Conductivity (G)the inverse of resistivityis determined from the voltage and current values according to Ohm’s law. [G=I/R+1(amps)/E(volts)]. The higher the resistance the lower the conductivity.
A complete conductivity measurement loop consists of sensors that are exposed to the solution and an analyser that interprets and displays the conductivity measurement.
The basic unit of conductivity is the siemensformerly called the mho. Conductivity is usually measured in microsiemens or micromhos. A millisiemen is equivalent to 1000microsiemens and a millimho is equivalent to 1000micromhos. Since there is no standard equation for determining the conductivity of all solutionsbasic information regarding the electrolyte being measuredhas to be entered into the analyser. Each electrolyte possesses a unique conductivity curve. Strong electrolytessuch as hydrochloric aciddissociate fully in waterexhibiting a higher conductivity value than a weak electrolytesuch as acetic acid.
For most electrolytesthe conductivity curve reaches a maximum value (at a given temperature) and then reverses its slopeusually forming a bell-shaped curve. Conductivity can be measured on the increasing (front slope) or decreasing (back slope) part of the curve. Howeverit cannot measure concentration in the region where the curve changes slope (the top or flat portion of a curve)or on both sides of a curvesince two different concentration values can exhibit the same conductivity.
Temperature also has a significant effect on conductivity in that as the temperature increases so does conductivity. For instanceif you have a solution of five percent sodium hydroxide and you measure it at room temperature (25°C)you will get a certain conductivity value (223mS/cm). If you increase the temperature of that same solution (eg 50°C)the conductivity will increase (320mS/cm)though it remains five percent by weight concentration.
To account for the variables of chemical properties and effects of temperaturemost conductivity systems are equipped with curve sets for specific chemical concentrations and temperature compensation relative to the specific binary solution. For instanceif you are working with sodium chloride (NaCl)you would need the chemical concentration curve for that specific ion and the corresponding temperature compensation curve for that ion species. Sever measurement inaccuracies can occur without these two working in tandem. Without the proper temperature compensation curvestemperature variations can cause inaccurate conductivity readingssuch as a significant change in the displayed value of percent concentrationwhen in realitythe concentration has not changed at all.
For exampleif a clean in place (CIP) application requires a 0to5percent concentration rangethe end user may need to know that the process concentration does not drop below 2.5percent. If it didthey may not be getting the desired cleaning results because of an overly diluted solution. Converselyif the concentration increases to 8 per centthey may be wasting chemicalwhich increases production costs.
While chemical and temperature compensation curves are readily available for common solutionscustom curves can be developed for many non-standard binary solution applications.
Contacting conductivity or not
Two methods of sensing conductivity to accommodate a broad range of process conditions are contactingand electrodeless. A contacting conductivity sensor uses metal or graphite electrodes in direct contact with the fluid being measured. One electrode is excited by a high frequency ac signalthe resistance is then measured across the two electrodes and converted to conductivity.
Electrodeless conductivity sensing uses multiple toroids that work together as sender-receiver. One set of toroids is energised by the analysercreating an induced current in the liquidand the other toroids sense the field created. As the conductivity variesthe size of the sensed signal varies proportionally.
Contacting conductivity (CC) sensors offer low detection limits and are usually used in applications with clean fluidssuch as water. Where the measured value is typically less than 10microsiemens per centimetreCC should be considered. These ‘pure water’applications range from steam condensate and feed water for boilers and turbinesto ‘ultra pure’ water (less than 1microsiemen/cm resistivity – RS) for semiconductor processing. While CC/RS sensors are typically more accurate for precise and sensitive measurement (below 10microsiemens/cm)they do have an inherent problem; sensor coating. The electrode surfaces must be maintained in pristine condition to provide accurate measurements. Anything that coatsfoulsor otherwise contaminates them adds resistancewhich lowers the displayed conductivity value. Contaminates can include mineralisationoilseven bubbles.
To assure accurate measurementthe CC/RS sensors must be inspectedcleaned and calibrated routinely. The maintenance schedule depends on the application and how often and seriously the sensors become coatedor how critical the measurement. It may be every couple of daysevery weekevery two weeksbut the consequences of neglect could be grave. It’s a lot easier and less costly to plan frequent maintenance and/or replace a contacting conductivity sensor than a turbine or boiler.
Alternativelyelectrodeless conductivity (EC) requires far less maintenance and is typically unaffected by coatingssuch as chemical films and algae growth. The two most common EC configurations are insertion (invasive)which is immersed in the liquid flowand flow-throughwhich is installed inand becomes a section ofthe process pipeline. The surface or material of an EC sensor is not critical to making the measurement. Insteada field develops around the sensor headand that field is what detects the resistance of the solution passing through. EC sensors can be used in applications with conductivity ranging from approximately 5 microsiemens to 2millionmicrosiemens (2000millisiemens). Newerlarge bore insertion and flow-through EC sensors can operate at a minimum full scale range of 0to50microsiemens/cm with demonstrated measurement capability at or below approximately 5microsiemens/cm. Electrodeless conductivity technology is predominant for demanding applicationssuch as acids and other aggressive materials and process conditions with temperatures up to 411°F and pressures to 300psi+.
While conductivity measurement is a mature and established technologyrecent advances in system capabilities and materials have improved and expanded applications in process industries. For instancethe Measurements & Instruments Division of Invensys Process Systems introduced Foxboro sensors made from PEEKa thermo-plastic material proven to be compatible with the widest array of process solutions. Invensys- Foxboro also pioneered the used of virgin PEEK (P-oxyphenylenep-oxyphenylenep-carbooxyphenylene)which is FDA compliant and 3A approved for its sanitary sensors. For standard industrial sensorsthe company uses glass-filled PEEK (polyetheretherketone)along with other alternate sensor materials for non-standard applications. These include PVDF (polyvinylidenedifluoride)PCTFE (polychlorotrifluoroethylene)Norylas well as borosilicate glassglass-filled Teflonvirgin polypropyleneand several other thermoplastic materials.
For sanitary applicationsInvensys has developed a Foxboro electrodeless conductivity flow-through sensor that is appropriate for the full gamut of applications and allows the process to be automated.
Conductive solution solution
Following are two applications where conductivity sensing proved to be the cost effective solution.
Sulphuric acid is one of the most important industrial chemicals worldwide. The highly corrosive properties of this clear and colourless fluid are critical in applications ranging from the production of fertilisers to removing oxides from iron and steel. But there is a subtlermore delicate side of sulphuric acidand that is precise production. A variation of 0.1 per cent in acid strength could corrode profits by increasing production and material costscompromising quality and violating emissions standards.
Producing sulphuric acid involves precisely combining sulphurair and water. A major international chemical processing company accomplishes this by passing air through an oleum tower where it combines with sulphur trioxide (SO3) and moisture to form sulphuric acid mist particles at 99+percent strength. While the typical strength is in the 99.6to99.2 percent rangethe company needs the ability to measure up to 99.9percent.
A Foxboro conductivity measurement loop proved to be the solution. The system includes electrodless sensors and an intelligent analyser that together measure acid strength by sensing changes in electrical conductivity. The sensor is installed in-line in a sample loop of the sulphuric acid towers. The analyser/sensor determines the conductivity and converts that to concentration by weightthen the analyser sends data back to the plant control systems. Based on these measurements compared to preset conductivity limitsthe control systems bring in water or acid to reduce or maintain strength. To meet the specific materials compatibility demands of this applicationInvensys provided a PFA Teflon-coated toroid headcombined with a Carpenter20 alloy wetted metal housingand Viton O-rings. Howeverfor even more demanding applicationssuch as high purity acid where no wetted metal is permittedother sensor selections are required … and available.
To take full advantage of the sensor’s resistant material and innovative designthe company worked closely with Foxboro engineers to develop a custom curve set. While the analyser came programmed from the factory to handle the 99.5percent to 93percent rangethe custom curve set allowed accurate measurement of 99.85-pluspercent.
With sulphuric acidthe higher the strengththe greater the profitability. This company has found that the reliability and robustness of both the sensor and analyser have allowed them to consistently produce high qualityand highly profitablesulphuric acid.
The second example involves automating production of a high-value organic solvent. As part of the processthe high-value solvent separates from a salt concentrated by product. A critical step in the process is draining the heavier aqueous salt solution layer while leaving the lighter solvent. This was traditionally done by qualified technicians who would observe the fluids emptying the tankrelying on sight and individual experience to try to stop the flow at the right momentto avoid losing the revenue rich chemical. Howevereven seconds of delay could result in the loss of thousands of dollars worth of profits quickly down the drain.
The solution to this balancing act proved to be the Foxboro 871FT flow through sensor. The saltwater byproduct has a high ionic concentrationwhich exhibits a strong conductivity compared with the organic compound that exhibits negligible conductivity. The non-invasive sensors measure the ionic content of the liquidsand instantly signals when a sudden and significant change in conductivity occurs. This triggers a signal that shuts the drain. The result is greater process efficiencyreduced maintenance costs and reduced worker interaction with chemicals.
J Kevin Quackenbush is Senior Conductivity Measurement SpecialistInvensys Process SystemsMeasurements & Instruments DivisionFoxboroMAUSA. www.invensys.com"